Gather all ingredients.
Prepare the Dipping Sauce: Whisk together mayonnaise, ketchup, relish, Worcestershire, garlic powder, and cayenne in a small bowl. Cover and refrigerate until ready to use.
Prepare the Onion Patties: Pour oil to a depth of ½ inch in a large cast-iron skillet; heat over medium-high. While oil heats, whisk together flour, cornmeal, sugar, baking powder, salt, garlic powder, and cayenne in a bowl.
Add onion and buttermilk and stir until combined.
A drop of batter should sizzle as it hits the oil, indicating it’s ready. Working in batches, spoon a scant ¼ cup of batter into the hot oil, and using a spoon, spread slightly into circles. Fry until golden brown on both sides, about 3 minutes total. Remove from pan to a wire rack set over paper towels and immediately sprinkle with salt to taste.
Serve hot with Dipping Sauce.
